What is the lowest radio frequency that can transmit the human voice?
Dave Yerzley
What is the lowest radio frequency that can transmit the human voice?
Maxwell’s equations, from which the wave equation for electromagnetic radiation can be derived, apply at all frequencies and wavelengths. However, long wavelengths present practical difficulties. 60 Hz power transmission lines radiate some energy, and this is enough to interfere with some of the measurements that atmospheric scientists need to make.
I’m going to assume analog transmission of voice and will not consider more sophisticated techniques or compression schemes.
The phone company established that intelligible speech could be transmitted with a bandwidth of about 3 kHz. Higher fidelity requires greater bandwidth. AM radio uses an audio bandwidth of about 10 kHz and FM radio has a wider bandwidth, up to about 15 kHz.
The most efficient antennas have dimensions related to the wavelength of the frequency being radiated, typically a half or quarter wavelength, depending on the configuration of the antenna. If the bandwidth is narrow, then the antenna will work well not only for the carrier frequency but also for the sidebands which carry the information. The lowest AM radio frequency, 540 kHz, transmits sidebands out to 10 kHz on either side of the carrier and so has a bandwidth which is about 4% of the carrier frequency. FM radio may have sidebands out to about 120 kHz on either side of the carrier frequency, so the bandwidth at the lowest FM signal, 88.1 kHz, is only 0.25% of the carrier frequency. Building an antenna which can accommodate these bandwidths is not a particular challenge. TV channel 2 uses frequencies from 54 to 60 MHz, for a bandwidth of about 10% and these antennas require a little more care to work well over that bandwidth.
FM radio has a wavelength of about 3 meters and TV channel 2 has a wavelength of about 5 meters. This means that antennas for these services can be relatively small. The wavelength for AM 540 kHz is about 550 meters, or about a third of a mile. Numerous techniques have been developed to make vertical antennas considerably shorter than a quarter wavelength which work reasonably well.
The lowest frequency which I am aware of in regular use for radio transmission (neglecting very low frequency transmissions used by the defense department) is 60 kHz for WWVB, an NIST station which transmits time information. This corresponds to a wavelength of 5000 meters, or about 3 miles.
The bandwidth necessary for voice transmission can be cut in half by transmitting only one pair of sidebands, referred to as single sideband (SSB) transmission. Usually the carrier is also omitted. One could imagine using SSB transmission with a carrier frequency of zero, and in this case SSB transmission could be achieved simply by connecting the output of an audio amplifier directly to an antenna. For many reasons, this is not practical to do. Among those are the very long wavelengths and the extreme bandwidth percentage required of the antenna.
So there’s no theoretical lower limit for the frequency which could be used to transmit voice information, but practical difficulties increase as the frequency becomes very low.
458_lowest_rf
10-03-2019